Pulmonary arterial hypertension (PAH) is a disease of the small pulmonary arteries marked by a progressive increase in pulmonary vascular resistance, leading to right heart failure and ultimately death. An imbalance between vasoconstriction and vasodilation coupled with vascular remodeling and overgrowth are hallmarks of PAH. Nitric oxide (NO) is known to play a protective role in PAH and endothelial failure and loss of NO signaling contribute to both human and experimental PAH. Recent work by the principal investigator has found that the matrix protein thrombospondin-1 (TSP1) blocks physiologic NO responses in vascular cells. Preliminary data suggests that TSP1, in binding to its necessary receptor CD47, inhibits activation of endothelial nitric oxide synthase (eNOS) and thus endothelial NO production, and that TSP1 and CD47 are dramatically upregulated in human and experimental PAH. Additional new data demonstrates that the TSP1-CD47 nexus stimulates vascular smooth muscle cell overgrowth in a PDGF-dependent manner in PAH. The central hypothesis to be tested in this proposal is that excessive TSP1 expression underlies PAH vasculopathy and that interaction with its cognate receptor, CD47, selectively regulates key second messenger pathways linked to microvascular vasoreactivity, growth, and remodeling. The present proposal will explore (i) the mechanisms behind upregulation of TSP1-CD47 in PAH, (ii) the implications this has on vascular tone and overgrowth and (iii) the effects that therapeutic interruption of the nexus has on preventing PAH and on ameliorating established disease.